1. Field of the Invention
The invention relates to an extrusion head having a central die core and outer die casing ring cooperating to define an annular exit gap or orifice for fabricating hollow extrudates. The cross-sectional dimension of the gap is uniformly changeable by axial displacement of the die core and/or the die casing ring relative to each other; and one or both of the die members is segmented with each segment adjustable to vary the cross-sectional dimension of the gap at prescribed points circumferentially about the gap.
2. Description of the Prior Art
Extrusion heads have been developed where the die gap of the exit die is changed in the circumferential direction at prescribed points to attain a specified distribution of wall thickness in the hollow extrudate. This is desirable, for example, when blow molding a preform of hollow extrudate between the segments of a blow mold cavity. There, when the extrudate is clamped by the mold segments, the extrudate will traverse different stretch paths in the area of the mold clamping points than in the remaining portions of the mold. When an extrudate of uniform wall thickness, the walls of the finished blown product will have variable thickness in relation to the extent to which the extrudate stretches at different locations within the mold.
By means of a specific distribution of wall thickness in the extruded preform, it has been possible to balance out weak or thin areas that would otherwise be created during the blow molding process. This has been accomplished by extruding the preform with a wall thickness of different dimensions in such a manner that the wall is thicker in areas of longer stretch paths and thinner in areas of shorter stretch paths. Thus, after the blow molding process has been completed, that is when the stretch of the material has terminated, the hollow body that is formed will have walls of equal thickness.
In one prior method, a flexible die casing ring has been placed adjacent the exit of the annular die, as disclosed in DT-OS No. 26 54 001. In order the change the annular gap of the die at prescribed points, this ring is deformed in the radial direction within its elastic range through the action of tension and pressure screws. According to another method disclosed in DT-PS No. 1 161 412, the die casing itself is elastically deformable in the radial direction at prescribed points. In order to avoid joints between the deformed die ring and the recesses of the housing, in which the ring is guided, a ring lining is here inserted between the ring and the hollow extrudate as disclosed in DT-OS No. 26 10 668.
Prior attempts at a solution require the application of considerable deformation forces to the casing die ring. The wall thickness of the ring must be sufficient to withstand the high mechanical and thermal die stresses created during the extrusion processes. Furthermore, such a deformable ring must be capable of springing back into its original circular shape when it is relieved from deformation forces. Consequently, for these reasons, the wall thickness of the die ring may not fall below a certain value.
A further disadvantage of prior ring constructions relates to possible warping. When a closed ring is deformed, the circumferential length of the ring remains constant. Pressing in at one point of the ring can simply cause an undesirable bulging out at another point, so that there is merely a change of shape of the ring rather than a controllable change in diameter.
The larger the volume of the container that is being fabricated by the blow molding process, the more material must be utilized. Consequently, there will be a larger aggregation of excess material at the critical points behind the mold separation seams formed by the clamped segments of the mold. For this reason, in order to constrict the die gap to form thin extrudate at the selected points, the associated ring areas must traverse comparatively large deformation movement at these same points. The heavier the design of the ring cross-section, the less is its deformation capability, so that the deformation that can finally be achieved is not sufficient to remove sufficient material at the desired extrudate points. Under extreme operating conditions, the ring can experience permanent material fatigue, which will cause the ring to break, or permanently deform, which renders the ring useless. With the prior art constructions, it is therefore only possible to provide for a few force application points for deforming the ring over its circumference.
In the prior art techniques for changing the die gap in the circumferential direction at prescribed points, the mode of behavior of the plastic in the flowable state has been a decisive factor. The assumption has been that smooth continuous extruding surfaces of the die gap were needed to create smooth thickness changes in the wall thickness of the extruded preform material. Consequently, the prior techniques provided for smooth transitions in the local displacement regions of the gap. The most favorable smooth transitions were produced by radial deformation of the die casing ring in the elastic range of the ring material. Therefore, the above-described disadvantages of limited constriction of the die gap and a limited number of constriction points about the die gap periphery had to be accepted.